Digester control systems



Nov. 25, 1958 A. E. BENNETT 2,861,884

DIGESTER CONTROL SYSTEMS Filed Feb. 21, 1956 O 0 WWW INVEN+EH=LZ T lg- 2 fl/YDEEW 25. BEN/V577 United States Patent 2,861,884 4 DIGESTER CONTROL SYSTEMS Andrew E. Bennett, Weymouth, Mass., assignor, by mesne assignments, to Worthington Corporation, New York, N. Y., a corporation of Delaware Application February 21, 1956, Serial No. 567,020 1 Claim. (Cl. 92-7) This invention relates to the control of pulp digesters and particularly concerns the relief of vapors in such manner that circulation of the digester contents is promoted and a cleaning action analogous to steam distillation is effected.

-As is well known to those skilled in the art in the manufacture of paper pulp, a digester is filled with wood chips of a character suited to the product and is then charged with cooking liquor until the major portion of the chips are covered, the liquor being in the form of an aqueous solution of chemicals suited to the process. Heat is then applied to the mass, either directly by the introduction of steam at the bottom of the digester with or without forced circulation, or indirectly by forcibly circulating the liquor from the digester to a heat exchanger and thence back to the digester, it being desirable with eithermethod to raise the temperature rapidly to a predetermined value which is maintained for a selected period of time.

In the past it has been the theory that when noncondensable gases accumulate at the top of the digester, the presence of air and noncondensables causes the temperature to be lower than the temperature corresponding to that of saturated steam at the existing pressure. It was the practise therefore to vent the air and other noncondensables in order to permit the steam to replace the noncondensables and thereby raise the temperature to that corresponding to the steam pressure. Thus the relief valve was arranged to open when the digester temperature dropped below that of steam at the existing pressure, and when steam replaced the noncondensables and the temperature returned to that corresponding to steam at the existing pressure, the relief was throttled to reduce the venting of vapors.

I have found that the prior practise of opening the re lief valve on decreasing temperature in respect to'steam temperature tends to disrupt the cooking process by interfering with the circulation of the cook mass during the initial stages of the cook. This is attributed to a condition wherein steam is admitted to the bottom of the digester at a rate in excess of the condensing rate, thereby creating sufiicient pressure differential between the bottom pressure and top pressure to lift the mass of liquor and chips to the top of the digester. When this occurs, the liquor which is relatively cool at the initial stages of the cook is in contact with the temperature sensing element causing the relief valve to be opened. The open relief valve lowers the pressure at the top of the digester, whereby sufiicient top and bottom differential pressure is maintained across the cook charge to tend to hold the charge at the top of the digester and to pack it tightly together, thereby virtually prohibiting the initiation of circulation. This condition necessitates the reduction of steam admission in order to allow the mass to settle down, thereby prolonging the cook period. Furthermore during the cook upset, an excessive quantity of unused pooking liquor is lost through the relief line. resulting in wastage of chemicals and interference with the normal cook process.

I have discovered that the upsets referred to above can be partially prevented and/or automatically removed before they become serious by closing the relief valve when the temperature at the top of the digester is below that corresponding to the temperature of steam at the existing pressure. By this means the relief valve is closed when the top temperature of the digester is below the corresponding steam temperature, thereby preventing the maintaining of a differential pressure across the charge suflicient to lift or hold the charge at the top of the digester. When circulation has been established, the top temperature rises to approach that of steam at the existing pressure, and in accordance with my invention, the relief valve is opened and steam is permitted to escape.

This promotes circulation in that steam which has not condensed in passing through the charge has completed the important function of promoting circulation. As the uncondensed steam rises through the charge, part of the motion of the steam bubbles is imparted to the charge to create circulation currents, and by relieving the uncondensed steam at the top of the digester, additional steam from the bottom of the digester rises to replace it.

I have found that the venting of quantities of uncondensed steam results in a cleansing action analogous to steam distillation, thereby removing high boiling wood chemicals, such as turpentine, terpenes and resinous materials which, if left in the digester, tend to delay the chemical action of the liquor in breaking down the wood structure. The combined efiects of the enhanced circulation and of the cleaning action result in much greater uniformity of the finished pulp. A less rigorous cooking procedure is required to break down the structure of knots and hard spots so that both overcooking of clear wood, and rejects resulting from undercooking of knots and hard spots, are substantially reduced. In actual practise, I have foundthat the washing qualities of the pulp have materially improved and that the tear-strength of the finished product has significantly increased. I have also found that the amount of chemicals required in the liquor has been reduced effecting a substantial saving.

In connection with the above, I have found it preferable to provide means for varying the relief valve opening when the top temperature is substantially equal to the temperature of steam at the existing pressure. This includes differential and ratio adjustments, whereby the relief valve opening is greater under these conditions when the digester pressure is low than the relief valve opening under the same conditions when the digester pressure is high. This compensates for the change in pressure drop across the relief valve, thereby providing maximum relief valve opening during that part of the cook period when the major portion of the wood chemicals are removed, and reducing the relief valve opening during the higher pressure stages of the cook. This feature of my invention is preferably utilized to prevent excessive relief of vapors after higher temperatures and pressures have been reached.

It is an object of this invention to govern the relief of digester vapors in a manner to promote circulation of the digester contents and to provide prompt appropriate control action to alleviate incipient upsets in the digester cooking process.

It is another object of this invention to provide increased vapor relief, thereby effecting the removal of higher boiling wood chemicals, such as turpentine, terpenes and resinous materials, by a process analogous to steam distillation, whereby pulping and washing are facilitated, and recovery of wood chemicals: is increased.

These and other objects of this invention will be more fully understood from the following description 3, when taken in connection with the accompanying drawings, wherein:

Fig. l is a view, partly in diagram, showing a vaporrelie-f controller withassociated relief valve, embodying this invention, as applied to a pulp digester of the direct steam heated type; and- Fig. 2 is a front elevation, enlarged and partly in diagram, of the-controller shown in Fig. 1.

Having reference to the drawings and particularly to Fig. 1, there is shown a pulp dig-ester, generally indicated at 10, which includes a body portion 11, a cone-shaped bottom 12, a dome 13, and a neck 14, mounted on thedome, having the usual detachable cover to'provide for charging the digester with wood chips and liquor, the normal liquor charging level being indicated by broken line 18. The bottom 12has connected therewith a pipe- 16' equipped with a hand valve 17 through which the finished pulp may be discharged at the end of the cook. 7

. The digester 10 is-provided-witha relief line 20 which is connected with the neck 14,. the neck being herein shown as including a removable screen 21 to prevent solid or semi-solid material from entering the relief line. Any means, notshown, for clearing the screen when required may be utilized as, for example, is shown in myapplication for Letters Patent of the United States, filed January 3, 1956; under Serial Number 556,958. Venting of vapors from the digester is varied by a relief valve 22 which is disposed in the relief line 20 and is herein shown as pneumatically actuated and respono sive to changes in output pressure from a controller, generally indicated at 25. As illustrated, the valve 22 is reverse acting, being provided with a diaphragm 26 which partially defines a pressure chamber 27 with which the output pressure from the controller is connected by means of a pipe 28. The diaphragm 26 actuates a valve member 29 by means of a valve stem 30, the valve member being preferably of the double seated type. Pressure in the chamber 27 is opposed by the usual spring 31. It will be understood that the valve 22 opens on an increase of pressure in the chamber 27 and closes on a decrease of pressure in said chamber.

The controller 25 may be similar to that shown in the United States patent to Robins, No. 2,585,347, with pressure-temperature differential response mechanism having a ratio adjustment incorporated therein. When so arranged, thecontroller referred to' will accomplish the objects of this invention when applied to a pulp digester to govern the relief of vapors therefrom. In order to simplify this description however, the controller shown in Fig. 2 is a partially diagrammatic illustration of mechanism suited to the purpose. The controller may be mounted in a case, indicated by broken lines 32, having the usual backplate 33 on which the mechanism is supported. This comprises differential pressure and temperature response mechanism which includes B'ourdon coils 34 and 35, a differential pressure-temperature setting device, generally indicated at 36', a ratio adjustment for said mechanism, generally indicated at 3'7, and a proportioning device for varying operating pressure to the relief valve 22, generally indicated at 38. The coil 34 is responsive to digester pressure and, as herein shown, connects with the bottom- 12 of thedigester by means ofa pipe 39. Any well known form of means, not shown, for providing reliable measurements of pressure may be utilized, such as a water back purge or equivalent device. Hydrostatic legs occurring in the digester and the connecting pipes by Which the controller 25 responds to pressure at the bottom of the digester, may becomp'ens'a'ted by means o'f a conventional Zero adjustment tobe hereinafter referred to. While I have shown the coil 34 responsive to pressure at the bottom of the digester and prefer this arrangement, pressure measurements may be taken elsewhere in the digester if desired. The con is responsive to t p e at 4. the top of the digester and for this purpose forms a part of a vapor pressure system which includes a capillary 40 and a bulb 41 provided with a volatile fluid having a vapor pressure curve similar to that of water. The bulb 41 extends through a suitable opening in the dome 13 of the digester and makes a pressure tight fit therewith. Each of the coils 34 and 35 has a fixed end 42 with which the pipe 39 and capillary 4t? are connected, respectively;

The pressure coil 34 has a shaft 43, disposed centrally thereof, with which the free end of the coil is in operative connection, it being understood. that the shaft is rotated by the coil in proportional relation to changes in digester pressure. Similarly the temperature coil 35 is provided with a central shaft 44 which is rotated by the coil in proportional relation with changes in temperature at the top of the digester. An index 45, adjustably secured to the shaft 43 of the pressure coil 34 by means of a clamping device 46, toprovidethe conventional Zero adjustment, cooperates with a scale 47, and an. index 50, connected with the shaft 44 of the temperature coil 35, cooperates with a scale 51. Coils 34 and' 35 are so arranged that when the top temperature of the digester is the same as the corresponding temperature of steam at the existing pressure, the indexes 45 and will occupy the same position on their respective scales. In actual practice, since a temperature curve is non-linear with a pressure curve, the non-linearity is compensated for by arranging the graduation of a. chart or scale to. effect this result. In order to simplify this description, the scales 47 and 5.?1 are graduated in terms of percentage of range, it being understood that the pressure range of coil 34 is suitable for digester cooking. and that the temperature range of coil 35 is similar thereto. Again in order to simplify this description, a third scale 54 is shown with which an index 55 cooperates to indicate the setting of the controller in a manner to be described.

The controller 25 functions to govern the relief of vapors from the top of the digester in response to the difference between the'measured top temperature and the temperature of saturated steam at the existing pressure throughout the cook period, and in the event that the top temperature drops below that of steam at the existing pressure, the relief valve 22 is throttled until the top temperature returns to the steam temperature, at which time the valve opens to vent digester vapors through the relief line 20. The degree of motion of the valve in response to the said difference is governed by the conventional proportional band adjustment to be hereinafter referred to. The coils 34 and 35 and the setting. index 55 are mechanically interconnected by a differential linkage. The setting index 55 is positioned on the scale 54 by means of the digester pressure coil 34 to indicate the control setting. For this purpose the setting index 55 is mounted on a fixed pivot 56 at its lower end and has a pivotal connection 57 with a link 58 midway between the ends of the said link. The link 58 has at one end a pivotal connection 59 with the upper end of a link 60 and at the other end is pivoted at 61 with a floating link 62. The link 60 has between its ends, a pivotal connection 63 with a sector plate 64, and at its lower end has an adjustable pivotal connection 65 with one end of a link 66 the opposite end of which is pivotally connected at 67 with the lower end of the pressure index 45. The lower end of the floating link 62 is pivoted at 68 to one end of a link 69 the other end of which has a pivotal connection 70 with the lower end of the temperature index 50. The floating link 62 is pivoted at 71, midway between its ends, to one end of a link 75, herein shown broken for clarity, by which the proportioning device 38 is actuated to vary operating pressure to the relief valve 22 in a manner to be described.

The floating link 62, is positioned around the lower pivot 68 by the pressure coil 34 in response to changes in digester pressure, and the said link is positioned around enemas the upper pivot 61 by the temperature coil 35 in response to changes in digester top temperature. ting index 55 is positioned on the scale 54 by the pressure index 45 of the coil 34 on a selected adjustable basis, to be described, each position of the setting index 55 will require an identical position of the temperature index 50 to position the pivot 71 at the midpoint of the proportional band. Thus the setting index 55 determines the center of the proportional band.

The proportioning device 38 includes a nozzle 76, a battle 77, and a bellows 78. The nozzle 76 receives operating fluid, such as air under regulated pressure, from a source, not shown, through pipes 79 and 80, the former having a restriction 81 of a capacity less than that of the nozzle so that the nozzle pressure is determined by the relative position of the nozzle and baflie. The bellows 78 has a fixed end 82 and a free end 83 to which a post 84 is rigidly secured. The interior of the bellows 78 connects with nozzle pressure in the pipe 80 by means of a pipe 85, and the pipe 80 in turn connects with the pipe 28 by which the nozzle pressure is communicated to the diaphragm chamber 27 of the relief valve 22.

Means for actuating the baffie 77 in respect to the nozzle 76 include an arc-shaped member 86 having at its lower end a pivotal connection 87 with the bellows post 84 and at its upper end a pivotal connection 88 with the link 75. The baflle 77 is pivotally connected at 89, between its ends, on a fixed post 90, one end of the baflle being adapted to cooperate with the nozzle 76 and being normally held in engagement therewith by a compression spring 91 disposed between the baflle and a fixed member 92. Means for positioning the bafile 77 in respect to the nozzle 76 is in the form of a baffle actuating member 93 rigidly secured to a rotatable arm 94 pivotally connected at its upper end 95 to a fixed bracket 96 and at its central portion having a pivotal connection 97 with one end of a link 98. The other end of the link 98 is connected by an adjustable pivot 99 with the arc-shaped member 86. The radial center of the areshaped member 86 is at the pivot 97 and the adjustable pivot 99 cooperates with a slot 100 in the arc-shaped member and is provided with locking means 101 for clamping the pivot at any selected position lengthwise of the slot. Movement of the adjustable pivot 99 towards the upper pivot 88 narrows the proportional band, and movement of the pivot 99 towards the lower pivot 87 widens the proportional band.

In operation, the pressure coil 34, through the links 66, 60 and 58 positions the setting index 55 on the scale 54 in accordance with changes in digester pressure. The temperature coil 35, through the link 69, varies the position of the lower pivot 68 of the floating link 62 about the upper pivot 61, the position of which is established by the setting index 55. Therefore as long as the top temperature of the digester coincides with the setting index 55, the center pivot 71 of the floating link remains fixed,

no motion is transmitted to the proportioning device 38 by the link '75, and the output pressure to the relief valve 22 remains constant, which pressure, according to the usual practise is at the middle of the output pressure range. However, if a difference occurs between the setting index 55 and the temperature index 50 so that the temperature index is below the setting index, the center pivot 71 of the floating link 62 is moved to the right as shown in Fig. 2, thereby positioning the link 75 to the right. This movement swings the arc-shaped member 86 clockwise around its lower pivot 87 and by means of the link 98, the baflie actuator retracts the battle from the nozzle. The resulting reduction in nozzle pressure permits the bellows 78 to contract and return the baffle to a throttling relation with the nozzle at a lower output pressure. The lower output pressure throttles the relief valve and reduces the relief of vapors from the digester. When the temperature index 50 again coincides with the setting index 55, the output pressure returns to a mid- Since the setrange value and thereby returns the relief valve 22 to its original opening. On the other hand, if a ditference occurs between the digester top temperature and the setting index 55 so that the temperature is above the index setting, the output pressure increases proportionally, thereby providing a proportional increase in relief valve opening and in vapor relief.

As previously referred to, the setting index 55 is positioned by the digester pressure index 45 on some selected adjustable basis. For this purpose, as herein shown, the sector 64 is utilized to introduce a diflerential setting between the setting index 55 and the pressure index 45. The sector 64 has a fixed pivotal connection 105 at its lower end and at its upper end is provided with a toothed arc-shaped section 106 which cooperates with a manually operable pinion 107. Thus when the sector 64 is in the mid position shown, the position of the setting index 55 on the scale 54 corresponds with the position of the digester pressure index 45 on the scale 47.

As shown in Fig. 2, the sector 64 is at its mid position of the range of adjustment, and the pivots 59 and 65 are equidistant from the pivot 63 on which the link 60 is mounted to provide equal leverage arms 108 and 109. Therefore the differential setting between the pressure index 45 and the setting index 55 is zero percent, and since the leverage arms 108 and 109 of the link 60 are equal in length, the position of the setting index 55 on the scale 54 coincides with the position of the pressure index 45 on the scale 47 at all pressures occurring in the digester throughout the cook period. Inasmuch as the proportional band is centered about the setting index 55, when the differential setting is zero percent and the leverage arms 108 and 109 of the link 60 are equal in length, the proportional band will be centered about the position of the pressure index 45. The center of the proportional band can be shifted above or below the position of the pressure index 45 by rotating the sector plate 64 clockwise or counterclockwise, respectively. A clockwise movement of the sector plate 64 shifts the position of the pivot 63 to the right, as viewed in Fig. 2, around the pivot 105, thereby rotating the setting index 55 to the right and swinging the floating link 62 clockwise around its lower pivot 68. This motion is transmitted by the link 75 to the proportioning device the output pressure of which is determined by the position of the said link. When the link moves to the right, the baflle 77 is positioned away from the nozzle 76, thereby reducing the pressure in the bellows 78. The resulting contraction of the bellows swings the arc-shaped member 86 clockwise about its upper pivot 88, thereby acting through thelink 98 to return the bafiie to a throttling relation with the nozzle at a lower output pressure. Conversely a motion of the link 75 to the left produces an increased output pressure. Thus by means of the differential setting provided by the sector plate 64, the center of the proportional band can be varied in respect to the position of the pressure index 45 on its scale 47.

While the control mechanism hereinabove described functions to provide prompt and appropriate control action to incipient upsets in the digester cooking process and effects a cleaning action on the pulp analogous to steam distillation to remove turpentine, terpenes, and resinous materials, a further refinement in the control action has proven advantageous. For this purpose I provide the ratio adjustment, generally indicated at 37, in combination with the differential adjustment. The ratio adjustment consists of the adjustable pivot 65 which cooperates with a slot 111 in the lower portion of the leverage arm 109 and may be clamped at any selected lengthwise position in the said slot by means of a clamping device 112. By this means an adjustable ratio between the leverage arms 108 and 109 is provided. When the pivot 65 is raised from the position shown in Fig. 2, wherein the leverage arms are shown as equal in length, and the leverage arm 109 is shortened so that its length 7 is less than that of the leverage arm 108, the movement of the setting index '55 over its scale is greater than that of the pressure index 45 over its scale for a given change in digester pressure, the difference in movement being determined, by the leverage ratio of the said arms. On the other hand, if the pivot 65 is lowered from the position shown and the leverage arm is thereby lengthened, the movement of the setting index 55 is correspondingly less than that of the pressure index 4-5. In practise I have found that the relief valve should be opened sufficiently to permit the free venting of vapors from the digester during the initial stage of the cook provided the top digester temperature is equal to the temperature of "steam at the existing pressure, since at this time in the cook period the pressure drop across the relief valve is rein tively small and vapor flow through the valve at a so lected opening is correspondingly small. On the other hand, with this condition of temperature and pressure, as the digester pressure increases, a corresponding increase in pressure drop across the relief valve occurs so that at maximum digester pressure the valve opening required to pass the said volume of vapors is considerably less than at the initial stage of the cook. I therefore provide a relief valve opening of sufficient selected capacity to permit the free venting of vapors at the initial stage of the cook when the positions of the indexes 45 and 50 correspond on their respective scales, and gradually reduce the valve opening during the pressure rise period when the positions of said indexes correspond, thereby compensating for the increasing pressure drop across the relief valve. This method of controlling the relief of digester vapors is provided by the differential setting 36 and ratio adjustment 37. If desired the volumetric relief fiow can be varied over the cook period by means of the ratio adjustment.

As has been described above, the function of the differential and ratio adjustments is to vary the position of the setting index 55 with respect to that of the pressure index 45 in that the dilferential adjustment may be used to shift the setting index above or below the pressure index by a selected amount, and the ratio adjustment may be used to vary the motion of the setting index with respect to the motion of the pressure index. I have found it desirable to adjust the setting index 55 at a selected value below the pressure index 45 at the start of the cook, and by means of the ratio adjustment the setting index is caused to overtake the pressure index as the digester pressure rises until at the maximum digester pressure the setting index is above the pressure index by a selected value. By this means, inasmuch as the proportional band is centered about the setting index 55, the output pressure and consequently the relief valve opening is governed by the position relationship between the setting index and the temperature index 50. As is the usual practise in control devices, mid-range output is provided when the positions of the setting index 55 and temperature index 50 coincide on the scale. When the temperature index 50 is above the setting index 55, the nozzle 76 is throttled to provide increased output pressure and relief valve opening. Conversely when the temperature index 50 is below the setting index 55, the baffle 77 is retracted from the nozzle to provide reduced out- 8 7 put pressure and relief valve opening. When adequate circulation is present in the digester so that saturated steam fills the top of the digester, the measured temperature and pressure are such that the positions of themdexes coincide on the percentage scales, since the temperature measuring system has been selected to have a vapor pressure curve substantially the same as that of water. Thus the existence of adequate circulation results in a coincidence of temperature and pressure index positions throughout the cook cycle. Therefore when adequate circulation is not present, the temperature will lag behind the pressure and throttle the relief valve until adequate circulation has been achieved. When this occurs, the temperature and pressure return to their normal relationship and the relief of vapors is increased.

It will be understood from the above that it is an object of this invention to provide control means for governing the relief "of vapors from a digester in a manner which will promote circulation, and to effect a cleaning action on the digester contents by a process analogous to steam distillation, wherein relatively nonvolatile compounds are removed at temperatures well below their boiling points at the existing pressure.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

In combination, a pulp digester having heating means for varying the digester pressure and a line for venting vapors from the top of the digester, a relief valve in said line, and control apparatusv having a source of power in operative connection with said relief valve, said control apparatus including an element responsive to digester pressure, an element responsive to digester top temperature, a proportioning device having a selected power operating range for varying power from said source to actuate said relief valve, mechanism operatively connecting said elements to vary the control setting of said temperature element in proportional relation to changes in digester pressure, said mechanism including means operatively connecting said temperature element with said proportioning device for varying power from said source to throttle the relief valve when the digester top temperature drops with respect to the control setting of the temperature element, and to open said valve as the digester top temperature rises with respect to the control setting of the temperature element, a differential device associated with said mechanism for varying the control setting of the temperature element in respect to the digester pressure as measured by the pressure element, and ratio adjustment mechanism associated with the diiferential device for varying the control setting provided by the differential device in proportional relation to changes in digester pressure.

References Cited in the file of this patent UNITED STATES PATENTS 

